Previous work using double-barreled chloride-selective microelectrodes indicates that the large absorptive cells lining the small intestine of Amphiuma actively accumulate chloride in vitro. It is proposed to determine the membrane mechanisms operating to transport chloride in these cells by further use of double-barreled chloride-sensitive microelectrodes in conjunction with net (isotopic) Cl flux measurements and total cellular chloride concentration measurements under a specific set of conditions. The specific conditions are designed to determine 1) the role, if any, of luminal sodium in chloride accumulation and chloride transport, 2) whether a HCO3-Cl exchange may be operating and, 3) what alterations occur, if any, in Cl accumulation and transport in villous absorptive cells when the tissue is exposed to a secretory stimulant, theophylline. These measurements will provide evidence for or against a role for the Na gradient in energizing Cl transport, a better understanding of the relationship between luminal HCO3 and Cl transport andthe role of the villous absorptive cells during chloride secretion produced by theophylline. Comparison of chloride activity and total cell chloride concentration will provide evidence for the relative mobility of the various "pools" of cellular chloride.